Activated protein C is a serine protease derived from the vitamin K-dependent protein, protein C. It has been implicated both as an inhibitor of blood coagulation and a stimulator of fibrinolysis. This proposal intends to investigate the anticoagulant properties of this enzyme by studying the mechanism by which it inactivates coagulation factor Va. This will involve the examination of the properties of another vitamin K-dependent protein, protein S which is a cofactor of factor Va inactivation. I plan to examine the mechanisms by which protein S interacts with activated protein C, which result in both altered anticoagulant activity and enhanced activity towards factor Va. Secondly, I plan to evaluate the role of phospholipid and cations as modulators of both the substrate specificity of activated protein C and its interaction with protein S. The long term goal of this research is to understand the mechanisms by which clot formation can be localized. One result of this type of research will be to develop tools that can be used to better understand the causes of the excess clotting observed in thrombosis.